The sr301 RX and savox servo could be causing a brown out even with the 7a bec.

As said above, even if your pack has juice left the low mah and low c rating could be tripping the lvc. Voltage sag. You would not have as much sag with a pack that has more mah and higher discharge capability.

Is your sensor wire sitting on the buttons????

4 things...
Make sure a wire is not near the buttons.
Add a glitch buster
Put current limiter very low like 20.
Set brakes to where they barely skid under full braking at the end of the straight.

All good? No issue?
Now go to 100 current limiter and full braking. Problems back? Its your lipo.